BONES OF WRIST AND ANKLE 197 merit of these bones, the distal bones only, or very nearly only, articulating with the corresponding bones in the upper series. In the modem types (cf. Fig. 113) there is, on the other hand, an interlocking, so that the bones of the distal series articulate with H FIG. 111.Series of metacarpals and metatarsals of Camelidae, to show secular and progressive increase in size. From left to right the species are Protylopus pete.rson.iy Poebrothefnawa, IcMatum* Gomphotherium stemferpi, Procamelus ocd- dentalis. F, Fore-foot; H, Mud-foot ; III, IV, tliird and fourth metapodiala, (After Wortman.) two of those of the proximal series. By this is produced, as it would appear, a much firmer foot, less liable to " give" under pressure, and thus more fitted for an animal that runs. It is the same principle as that adopted in the laying of bricks. The actual stress and strain of impact has been held responsible for those changes. An equally ingenious and possibly truer explanation of the undoubted facts has lately been advanced by Mr. W. D.